1. Field of the Invention
This invention relates to gas-fired appliance installations, and more particularly to a control system for such installations which eliminates the need for a conventional draft hood, while satisfying the purposes of the draft hood.
2. Description of the Prior Art
For many years, a draft hood has been a part of gas-fired appliances such as furnaces, water heaters, and the like. The draft hood is a fitting or device which is placed in and made a part of the flue pipe or vent stack, which vents the products of combustion from the firebox of the appliance to a chimney. One purpose of the draft hood is to provide for the ready escape of the combustion products in the event of no draft, back draft, or stoppage beyond the draft hood. The draft hood also neutralizes the effect of stack action of the chimney upon the operation of the appliance, and prevents back drafts from entering the appliance.
Draft hoods, which came into general use in an era of low cost energy, are very wasteful of energy. For example, in a furnace installation, the draft hood provides a massive leak to the external environment, typically a basement room in which the furnace is located. The draft hood permits air at room temperature to freely enter the flue through the draft hood opening which is typically over four times the cross-sectional area of the flue pipe. The draft hood also allows a back draft to simply blow out into the room, and should the flue become blocked, the products of combustion may also be exhausted into the room. Excessive stack draft is "short circuited" by the massive leak so that it is not seen by the firebox.
A further consideration is that when a gas furnace is not firing, a great deal of warmed room air is swept into the draft hood and up the chimney by the stack effect of the chimney itself. Warming this volume of "lost" room air requires fuel expenditure. Systems have been proposed to lessen stack losses by using a motor driven stack damper to close the vent stack when the furnace is off and to open the vent stack just before the furnace is turned on. However, when the furnace is on, this stack loss is even greater due to the increased stack effect of hot flue gasses. Accordingly, it would be desirable to have a furnace control system which satisfies the purposes of a conventional draft hood and eliminates the need for such draft hood.
A secondary energy loss in conventional draft hoods occurs due to the dilution of the flue gas and the resultant cooling of the stack gasses. It has been long established that the temperature of stack gasses issuing from the top of a chimney must be kept above approximately 250.degree. F. in order to avoid condensation of moisture laden with acids. When the flue gas from a furnace passes through the portion of vent in which a draft hood is provided, the hot air is diluted with cooler room air so that the temperature is reduced by over 200.degree. F. This is reported, for example, in Underwriters Laboratories Bulletin of Research No. 51, pages 62-63. If the draft hood is eliminated, as with the present invention, the system will have additional heat capacity. That is, a considerable amount of extra heat can be recovered and supplied to the heated space resulting in improved operating efficiency for the furnace. Stated in another way, the use of a draft hood requires that the furnace heat exchanger extract 200.degree. less from the flue gasses than would be allowable without the draft hood. Thus, a system which eliminates the need for a conventional draft hood would both minimize the loss of warmed room air and would permit use of increased heat exchanger capacity to the system.